High-speed detectors and detector arrays are required for telecommunication systems, for high-capacity local area networks, and for instrumentation. For chip-to-chip or processor-to-processor data exchange, optical interconnects will eventually replace today's electrical methods. Many such approaches are being explored. As the number of detectors required for each application increases, low material cost, mature fabrication technology and compact design present many advantages. Compared to detectors requiring surface normal coupling, waveguide detectors are simpler to assemble.
Traditionally photodetectors with high speed and high quantum efficiency have been limited to materials with direct band-gap, such as III–V compounds. It is desirable to fabricate such devices with silicon-based materials, to benefit from the low cost and mature silicon technology. Because the light absorption in the near IR range for silicon (Si) is low it is necessary to uncouple the optical path length (quantum efficiency factor) and the distance between electrodes (speed limit factor).
Metal-semiconductor-metal (MSM) detectors are often proposed as an alternative for high-speed operation, as the speed is primarily limited by the transit time between electrodes. The distance between photodetector electrodes can be scaled down through fine line lithography. MSM detectors are described in the following publications: “140 GHz metal-semiconductor-metal photodetectors on silicon-on-insulator substrate with a scaled active layer”, M. Y. Liu, E. Chen, and S. Y. Chou, Appl. Phys. Lett. 65(7), p. 887, 1994; “High-speed metal-semiconductor-metal photodetectors fabricated on SOI-substrates”, K. Honkanen, N. Hakkarainen, K. Määttä, A. Kilpelä and P. Kuivalainen, Physica scripta T79, p. 127, 1999; “Comparison of the picosecond characteristics of silicon and silicon-on-sapphire metal-semiconductor-metal photodiodes”, C. C. Wang, S. Alexandrou, D. Jacobs-Perkins, and T. Y. Hsinag, Appl. Phys. Lett. 64(26), p.3578, 1994. The contents of the aforementioned publications are incorporated here in by reference.
To increase the optical path length, and therefore the quantum efficiency, several approaches have been explored. Waveguide geometries, often formed on silicon-on-insulator (SOI) substrates as silicon ridges, have been used, as described in “SOI waveguide GeSi avalanche pin photodetector at 1.3 μm wavelength”, T. Yoshimoto, S. G. Thomas, K. L. Wang and B. Jalali, IEICE Trans. Electron., E81-C(10), p. 1667, 1998, and “Near-infrared waveguide photodetectors based on polycrystalline Ge on silicon-on-insulator substrates” G. Masini, L Colace, G. Assanto, Optical Materials 17 (2001) 243–246, incorporated here in by reference. The collection of carriers is accomplished either by forming MSM structures on the surface of the ridge waveguides, or by grown-in pin structures.
Another approach is to use vertical cavity structures, which consist of a thin absorbing layer sandwiched between two mirrors. In the past, vertical cavities have been used successfully to build high speed, high quantum efficiency photodetectors. The mirrors were made of dielectric/silicon or Si/SiGe multi-layers through deposition. This method is described in the following publications: “Selective epitaxial growth Si resonant cavity photodetector”, G. W. Neudeck, J. Denton, J. Qi, J. D. Schaub, R. Li and J. C. Campbell, IEEE Photo. Technol. Lett. 10(1), p. 129 (1998) and “Si/SiO2 resonant cavity photodetector”, D. C. Diaz, C. L. Schow, Jieming Qi, J. C. Campbell, J. C. Bean and L. J. Peticolas, Appl. Phys. Lett. 69(19), p. 2798 (1996).
Buried silicide layers, embedded by wafer bonding or implant, have also been used as mirror material for vertical cavities, as described in: “A vertical cavity longwave infrared SiGe/Si photodetector using a buried silicide mirror,” R. T. Carline, D. A. O. Hope, V. Nayar, D. J. Robins and M. B. Stanaway, Technique Digest of IEDM'97, p. 36.1.1 (1997) and in “Fabrication of ultra-fast Si-based MSM photodetector,” M. Löken, L., S. Mantel and Ch. Buchal, Electron. Lett. 34 (10), p. 1027, 1998. The contents of the aforementioned publications are incorporated here in by reference.
However, the devices described in the prior art present several limitations described below.
In the case of MSM detectors, only the photocarriers generated near the electrodes (i.e. near the surface) can be collected through drift in the electric field. For carriers generated outside the electric field, a portion of the carriers can be collected through diffusion rather that drift, resulting in a reduction in speed. Forming MSM structures on SOI substrates with thin silicon layers may eliminate this problem. However, the responsivity becomes very low.
Waveguide structures with vertical pin junctions require a specific layer sequence, therefore are costly to make and can limit the possibilities for component integration.
For the vertical cavity structures, light is coupled in from the surface normal direction, and these detectors operate only at a set of discrete resonant wavelengths. The resonant wavelength is determined by the distance between the two mirror surfaces. To obtain high quantum efficiency at the designed wavelength, stringent control in layer thickness is required, which often is a challenge in itself.
In view of the potential advantages of a Si-based photodetector, it would be very advantageous to provide a relatively simple Si-based photodetector with high quantum efficiency and fast response. This application discloses such is a photodetector.